1. Field of the Invention
This invention relates generally to thermoplastic belting, more particularly to belting with wear-resistant, anti-static fabric on a surface, and specifically to a drive belt with fabric cover containing silver-coated polymeric fiber.
2. Description of the Prior Art
Thermoplastic polyurethane (“TPU”) drive belts are used for transport applications, as well as for power transmission, motion control, and timing applications. TPU belts may be in the form of flat belts, toothed belts, V-belts, multi-v ribbed belts or other special profiles. They are typically electrically insulating. It is sometimes desirable that the belts not retain electrostatic charges, i.e., that the belts have anti-static properties.
Anti-static properties generally include one or more of the following characteristics: surface resistance lower than 108 ohms; volume resistance lower than 109 ohms; and ground bleeder resistance lower than 106 ohms/meter. Standards such as BS PD CLC/TR 50404:2003, DIN EN 13463-1, and IEC 60079-0 provide information on the avoidance of hazards due to static electricity.
U.S. Pat. No. 4,767,389 discloses a flat, plastic-covered-textile belt with anti-static properties arising from either an electrically conductive filament in the threads of the textile supporting element or an electrically conductive layer between the textile support and plastic covering. The conductive filament may be metal or carbon fiber, and the conductive layer may be soot-containing plastic. The plastic covering may be thermoplastic polyurethane. U.S. Pat. No. 7,328,785 teaches a conductive timing belt having a conductive layer of thermoplastic on the tooth surface. The thermoplastic may be conductive from the use of conductive microfibers, graphite or carbon black mixed therein.
U.S. Pat. No. 6,228,448 teaches use of an electrically conductive elastomeric surface ply which is preferably doped with a sufficient amount of carbon black or other conductive additives to give the outer ply or entire endless belt a surface resistivity of less than about 1014 ohm/square.
Metal chains are sometimes used in transport or conveying applications requiring conductivity or anti-static behavior, but the associated lubrication problems and noise are undesirable in many situations.
U.S. Pat. No. 5,417,619 teaches a covering canvas impregnated with an anti-static rubber composition based on conductive carbon black. An unwanted side effect of such coatings is a decrease in the abrasion resistance of the fabric resulting in rapid loss of the anti-static effect during use. The resulting abraded particles can be detrimental to nearby electronic or electrical components or systems. U.S. Pat. No. 5,351,530 makes use of such loss of conductivity to indicate the state of wear of a conductive-rubber-coated fabric.
It is known to incorporate so-called conductive carbon black or graphite in rubber and plastics to provide anti-static properties to the rubber or plastic composition. These compositions also generate undesirable abrasion products.